George Will(George Will's column is distributed by the Washington Post Writers Group.)

An axiom. When voters watch a presumptive presidential nominee considering this or that running mate, they think: What if the president dies?

When the presumptive nominee considers this or that running mate, he thinks: What if I live?Which brings us to the dotty idea that Barack Obama should choose to have Hillary Clinton down the hall in the West Wing, nursing her disappointments, her grievances and her future presidential ambitions while her excitable husband wanders in the wings of America's political theater with his increasingly Vesuvian temper, his proclivity for verbal fender benders and his interesting business associates.

That this idea survived her off-putting speech Tuesday night, after Obama won the right to choose a running mate, is evidence that many Democrats do not fathom the gratitude that less-blinkered Americans feel for Obama because he has closed the Clinton parenthesis in our presidential history.

After some of the boilerplate geographic pitter-patter that today's candidates consider Periclean eloquence (" ... from the hills of New Hampshire to the hollows of West Virginia ... "), she obliquely but clearly identified herself as the person who would be "the strongest candidate and the strongest president" and, pointedly, the person most ready to "take charge as commander in chief."

There is a fine line between admirable tenacity and delusional denial, and Clinton tiptoed across it.

Obama's choice of a running mate will be the first important decision he makes with the whole country watching, so it will be a momentous act of self-definition.

If he chooses her, it will be an act of self-diminishment, especially now that some of her acolytes are aggressively suggesting that some unwritten rule of American politics stipulates that anyone who finishes a strong second in the nomination contest is entitled to second place on the ticket.

Behind the idea that Obama should run in harness with Clinton is this wobbly theory:

Because the Republican Party is in such bad odor, if you unify the Democratic Party, that will suffice to win the election, and she is a necessary and sufficient catalyst of unity.

But she is neither.

She would be a potent unifier of John McCain's party, thereby setting the stage for exactly what the nation does not need, another angry campaign of mere mobilization rather than persuasion.

Surely she, the most polarizing Democrat, is NOT the only Democrat who can help Obama appeal to the voters who rejected him in Kentucky and West Virginia.

And as his running mate, she would nullify his narrative.

The candidate embracing the "future" should not glue himself to Washington circa 1999.

Someone promising to "turn the page" should not revert to an earlier chapter.

Someone whose mantra is "change" should not embrace her theme of restoration -- that the 1990s were paradise and Democrats promise paradise regained.

She, whose experiences as First Spouse have not impressed Obama as acquisitions of national security expertise, would not help him deflect McCain's predictable attack on his thin curriculum vitae.

And the more she seems to be pushing Obama to choose her, the more resolutely he must resist.Otherwise, at the beginning of a contest in which McCain will portray him as a flimsy figure, Obama will define himself as someone who can be pushed around.

On the eve of the battle of Trafalgar, Admiral Nelson, addressing his captains on the HMS Victory, picked up a fire poker and said:

It does not matter where I put this -- unless Bonaparte tells me to put it a particular place. Then I must put it someplace else.

Is Obama Nelsonian?

Selecting vice presidential candidates has recently become more serious than it was when Richard Johnson became Martin Van Buren's running mate in 1836 partly on the strength of the slogan "rumpsey dumpsey, rumpsey dumpsey, Colonel Johnson killed Tecumseh," a distillation of the unsubstantiated story that he personally killed the Shawnee chief at the Battle of the Thames in the War of 1812.

In 1964, Barry Goldwater did not reassure queasy voters when he said that one reason he chose to run with Bill Miller, an obscure upstate New York congressman, was that Miller annoyed Lyndon Johnson.

And remember the frivolousness that produced Maryland Gov. Spiro Agnew as Richard Nixon's running mate in 1968 and Missouri Sen. Tom Eagleton as George McGovern's in 1972.

Clinton, having risen politically in her husband's orbit, is a moon shining with reflected light.

Were Obama to hitch himself to her, he would reduce himself to a reflection of a reflection.

When by accident I ran into Zbigniew Brzezinski at a dinner Tuesday night, just before Barack Obama's victory speech, the first thing he did was to issue a personal warning about Hillary Clinton's continuing ambition.

"The worst thing Obama could do," he told me, "would be to allow her to have control of the vice presidency."

Then he issued an ironic aside about "family matters," adding, "And we'd have HIM hanging around all the time, too!"

Because Dr. Brzezinski is the closest thing we have these days to a wise man in the nation's capital -- and because he has been in the inner political circles of every possible sort ever since he served as Jimmy Carter's national security adviser in the late '70s -- his words are often uncomfortably prophetic.

Americans who thought they were witnessing a historic moment through the dignified and moving victory speech by Obama could be forgiven for wondering if they were actually watching and hearing two victory speeches.

If instead of the moment of the first black American about to be enthroned, they were about to see the first woman presidential candidate to win as well?

In the end, Dr. Brzezinski had it about right.

Obama's ideas for changing America would indeed be endangered if he were to accept Hillary as his running mate, simply because, given her character and aggressiveness, she would do everything possible to wrest power from him, serving more as an eternal challenger than as a supporter. (And her husband would be hanging around the White House again.)

Hillary is, and has been from the beginning, the candidate of the angry in America.

Barack is the candidate of the hopeful.

Indeed, part of the change he wants in America is not only from the Bush presidency but from what was well under way under the Clintons.

The kind of people he has wanted in his campaign and in a potential administration surely preclude both Clintons. Obama wants team players -- has anyone, anywhere, anytime, ever described either Clinton as such?

Obama has also said he does not want drama queens or kings -- case closed.

Even their appearances and styles are different.

Hillary is pugnacious, defiant, ready to roll and ready to fight. She will embellish her programs, even the admirable ones, in whatever kind of populist polish is available.

Look at her campaign: It has been marked by mismanagement, by demagoguery and by overspending, while her campaign staff members are described by insiders as themselves constantly wounded by infighting and typically '60s contentiousness.

Obama, on the other hand, is ever the paragon of gentility and gentlemanliness, and his potential sins are those of passivity.

Despite all of these differences, I still think that this campaign has been a promising one.

So let's try to get Hillary and Bill to agree about who has really won the Democratic nomination.